New Publication: Time-domain imaging of curling modes in a confined magnetic vortex and a micromagnetic study exploring the role of spiral spin waves emitted by the core

Congratulations to David Osuna, whose paper, ‘Time-domain imaging of curling modes in a confined magnetic vortex and a micromagnetic study exploring the role of spiral spin waves emitted by the core’, was published this month in Physical Review B. David has recently finished his PhD in the CDT and is now a postdoctoral research fellow at University of Exeter, as part of the Electromagnetic and Acoustic Materials Group (EMAG).

David explains the paper’s topic:

“This was a very fruitful collaboration with Dr. Paul Keatley, finally published after about 3 years of hard work!

Generally speaking, we have modeled and ‘filmed’ oscillations of the atomic magnetic spins in microscopic magnets and related them to other dynamics revealed from simulations. Understanding this type of dynamics as a whole is key to design spintronic devices, that may be essential for processing information in quantum computers, for example.

Abstract

The curling spin wave modes of a ferromagnetic vortex confined to a microscale disk have been directly imaged in response to a microwave field excitation using time-resolved scanning Kerr microscopy. Micromagnetic simulations have been used to explore the interaction of gyrotropic vortex core dynamics with the curling modes observed in the region of circulating in-plane magnetization. Hybridization of the fundamental gyrotropic mode with the degenerate, lowest frequency, azimuthal modes has previously been reported to lead to their splitting and counterpropagating motion, as we observe in our spectra and measured images. The curling nature of the modes can be ascribed to asymmetry in the static and dynamic magnetization across the disk thickness, but here we also present evidence that spiral spin waves emitted by the core can influence the spatial character of higher frequency curling modes for which hybridization is permitted only with gyrotropic modes of the same sense of azimuthal motion. While it is challenging to identify if such modes are truly hybridized from the mode dispersion in a confined disk, our simulations reveal that spiral spin waves from the core may act as mediators of the interaction between the core dynamics and azimuthal modes, enhancing the spiral nature of the curling mode. At higher frequency, modes with radial character only do not exhibit marked curling, but instead show evidence of interaction with spin waves generated at the edge of the disk. The measured spatiotemporal character of the observed curling modes is accurately reproduced by our simulations, which reveal the emission of propagating short-wavelength spiral spin waves from both core and edge regions of the disk. Our simulations suggest that the propagating modes are not inconsequential, but may play a role in the dynamic overlap required for hybridization of modes of the core and in-plane magnetized regions. These results are of importance to the fields of magnonics and spintronics that aim to utilize spin wave emission from highly localized, nanoscale regions of nonuniform magnetization, and their subsequent interaction with modes that may be supported nearby.

Fig. Time sequence of a radial-azimuthal spin wave mode simulated and experimentally imaged in a 2 micrometres diameter, 40 nm thick Permalloy disc with an in-plane RF excitation field at 10.24 GHz. Timestep is approximately 24 ps.

New Publication: An in situ investigation of the thermal decomposition of metal-organic framework NH2-MIL-125 (Ti)

Zahid Hussain

Congratulations to Zahid Hussain for his new paper, ‘An in situ investigation of the thermal decomposition of metal-organic framework NH2-MIL-125 (Ti)’ , recently published in Microporous and Mesoporous Materials.

Zahid explains the paper’s findings:

Metal-organic frameworks (MOFs) are exceptionally porous and highly crystalline coordination polymers. Since the late 1990s, MOFs have been intensively investigated for a large variety of applications such as gas separation and storage, energy storage and conversion, batteries, fuel cells, optoelectronics, sensing, supercapacitors, drug delivery and catalysis. However, many key questions need to be answered to optimize the synthesis of these materials for industrial-scale applications. In this study, we present an in-situ investigation of thermal conversion of a titanium-based MOF, NH2-MIL-125(Ti) under an inert atmosphere. In situ thermal analysis of NH2-MIL-125(Ti) reveals the presence of 3 defined stages of thermal transformation in the following order: phase-pure, highly porous, and crystalline MOF → intermediate amorphous phase without accessible porosity → recrystallized porous phase. The three stages occur from room temperature till 300 °C, between 350 and 550 °C and above ∼550 °C respectively. The derived disc-like particles exhibit a 35% volume shrinkage compared to the pristine MOF precursor. Highly crystalline N and/or C self-doped TiO2 nanoparticles are homogeneously distributed in the porous carbon matrix. The original 3D tetragonal disc-like morphology of the NH2-MIL-125(Ti) remains preserved in derived N and/or C doped TiO2/C composites. This study will provide an in-depth understanding of the thermal conversion behaviour of MOFs to rationally select and design the derived composites for the relevant applications.

The crystalline structure of Ti-MOF, NH2-MIL-125(Ti) and mechanism of its thermal decomposition.

 

 

We would also like to congratulate Zahid on passing his viva, and wish him the best of luck for his future career.

New Publication: Electrical Detection of DC Spin Current Propagation Through an Epitaxial Antiferromagnetic NiO Layer

Congratulations to third year PGR David Newman, whose paper ‘Electrical Detection of DC Spin Current Propagation Through an Epitaxial Antiferromagnetic NiO Layer’ has recently been published in IEEE Transactions on Magnetics.

David explains this work and the impact of its findings:

Spin currents (net transfer of spin angular momentum) have been suggested as

David Newman

potential successors to charge currents in areas like magnetic data storage. A spin current can be generated by a mechanism known as ‘spin pumping’ whereby a ferromagnetic (FM) layer is excited into resonance and then ‘pumps’ a spin current into an adjacent nonmagnetic (NM) layer. Recently, work has even found that antiferromagnetic (AFM) layers could even be used to amplify the amount of spin current produced.

 

The main issue comes with the difficulty in observing a pure spin current. Current techniques mainly consist of detecting the spin transfer torque exerted on an additional FM layer (AC spin current). Alternatively, the DC spin current can be observed by the transverse charge current generated by a spin current propagating through a heavy metal (like Pt) via the inverse spin Hall effect (ISHE).

 

In our work, we measure the ISHE on a sample with an AFM layer where the AC spin current has already been detected (see separate publication: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.217201) and, with some experimental considerations, extract the DC spin current. By showing AC and DC spin current components can be observed in the same sample, this work provides the pathway to a more complete perspective of spin current propagation through an AFM layer which is important in the development of spintronic technologies in areas such as magnetic data storage.

New Publication: Bimetal-organic framework derived multi-heterostructured TiO2/CuxO/C nanocomposites with superior photocatalytic H2 generation performance

Congratulations to final year PGR Zahid Hussain for his new publication ‘Bimetal-organic framework derived multi-heterostructured TiO2/CuxO/C nanocomposites with superior photocatalytic H2 generation performance’, which was recently published in Journal of Materials Chemistry A (JMCA).

In this work, In-situ formation of p-n heterojunctions between TiO2 and CuxO in heteroatoms-doped carbon nanocomposites and their applications in photocatalytic H2 evolution were demonstrated. One-step pyrolysis of bimetal-organic-frameworks NH2-MIL-125(Ti/Cu) in water steam at 700 ºC forms phasejunction between nitrogen/carbon co-doped anatase and rutile TiO2, accompanied by the formation of CuxO heterostructures. Moreover, p-n heterojunction is also formed between TiO2/CuxO nanoparticles. These multi-heterostructures are embedded in N-containing and hydrophilic carboxyl functionalised carbon. The optimised TiO2/CuxO/C composites absorb more visible light and offer multiple pathways for photoinduced electrons and holes migration. Also, these nanocomposites provide increased active sites for photocatalytic reactions. Without loading expensive noble metals, TiO2/CuxO/C nanocomposites exhibited superior photocatalytic H2 generation activity of 3298 µmol gcat-1 h-1 under UV-Visible light, 40 times higher than commercial TiO2. This work offers a simple approach to fabricate novel photocatalytic nanocomposites for efficient H2 generation.

Zahid’s previous publications include a translation of Carlo Rovelli’s “Seven Brief Lessons on Physics” into Urdu and ‘Surface functionalized N-C-TiO2/C nanocomposites derived from metal-organic framework in water vapour for enhanced photocatalytic H2 generation’ , of which he was lead author, published in Journal of Energy Chemistry.

Ned Taylor passes his viva!

Ned Taylor

Congratulations to CDT student Ned Taylor, who has just finished his PhD. His thesis was titled ‘Ab Initio Exploration of Interface Structures and Their Properties’.

Ned published five papers during his time as part of the CDT and presented at national and international conferences. He was also involved with the Metabuddies scheme- an outreach scheme led by our PGRs, who visit local schools to engage their students in physics and engineering.

Ned looks back on the experience of undertaking his PhD:

It is finally over. I have written my thesis, had my viva, completed my corrections and have been awarded my doctorate. Yesterday I had a final trek up to the Uni as a PhD student to pick up a copy of my thesis that I had printed for prosperity. It is weird to think that it is all over now; below are some of thoughts on my time as a postgraduate researcher.

A PhD is a unique experience for everyone. For me, it was a wonderful time and, although sometimes stressful, it was always worth it. I have relished in the opportunity to conduct research and the freedom to choose my path of study. I have learnt more about the materials that surround us than I had ever expected. The work was demanding, but that was never an issue as the topics were always fascinating. It was amazing going to conferences and have people attend talks to listen to the research that I conducted. Writing scientific papers was often rather tedious, but the outcome was definitely worth it and really helped in the process of writing my thesis.

I have learnt a lot throughout my PhD. In addition to the science, the courses offered by the CDT were, overall, very rewarding and useful. I learnt how to present, how to understand and communicate with others in a team, how to manage projects, how to put myself forward and highlight my skills. I feel that these skills will be vital for progressing beyond the PhD.

The community of PhD students in the CDT offered the chance to forge good friendships. Going on this journey with other PhD students helped me to relax and enjoy the experience. I am also extremely grateful for the support and guidance that my supervisors, Steve Hepplestone and Eros Mariani, have given me throughout the past four years.

As became a theme in the Hepplestone research group near the end of my PhD (before multiple lockdowns, that is), I shall summarise this chapter of my life with a haiku:

DFT was fun
Lots of time spent fighting tech
PhD is done

With my PhD complete, I am now transitioning over into the field of computer science. I have just started a postdoctoral research fellow position at University of Exeter, looking at ways of automating transport models for use in reducing the carbon impact of traffic systems.

We wish Ned all the best with his new role.

Below is a list of Ned’s publications, and conferences he has presented at:

Ned has co-authored the following publications:

2020

2019

2017

Ned has presented at the following conferences:

 

New Publication: Tunable pseudo-magnetic fields for polaritons in strained metasurfaces

Congratulations to final year PhD student Charlie-Ray Mann whose theory paper, entitled ‘Tunable pseudo-magnetic fields for polaritons in strained metasurfaces’, has been published in the prestigious journal Nature Photonics.

The work has also been featured on the University of Exeter’s news website.

Charlie-Ray Mann, the lead scientist and author of the study, explains:

Charlie-Ray Mann- lead scientist and author of the paper published in Nature Photonics

The interaction between charged particles and magnetic fields gives rise to some of the most fascinating phenomena in physics, ranging from the beautiful Aurora Borealis to the famous quantum-Hall effect. Unfortunately, because photons do not have an electric charge they are inert to real magnetic fields as they do not experience a Lorentz force.

Taking inspiration from graphene physics, we have shown that you can generate `artificial’ magnetic fields for light by distorting honeycomb metasurfaces in a specific way. These distortions generate a ‘synthetic’ Lorentz force which can deflect the surface polaritons into effective cyclotron orbits, and for larger distortions one can also observe Landau quantization of the polaritons — phenomena once thought to be exclusive to charged particles.

However, the main drawback with this approach is that to change the artificial magnetic field one is usually required to modify the distortion in the lattice. This is extremely challenging, if not impossible to do with photonic structures, hindering our ability to tune the artificial magnetic field after the structure has been fabricated — that is, the artificial magnetic fields are usually fixed by design.

In this theoretical work we have proposed an alternative mechanism to tune the artificial magnetic fields, which requires no change to the metasurface distortion. By exploiting the hybrid light-matter character of the surface polaritons, we show that one can tune the artificial magnetic field by modifying the real electromagnetic environment surrounding the metasurface.

Specifically, we’ve shown that by embedding the metasurface inside a photonic cavity or waveguide, one can tune the artificial magnetic field by modifying a single external parameter: the cavity width. In fact, we’ve even demonstrated that you can switch off the artificial magnetic entirely at a critical cavity width, without having to remove the distortion in the metasurface — something that is impossible to do in graphene or any system that emulates graphene.

Using this new mechanism you can bend the trajectory of the polaritons using a tunable Lorentz-like force that can be switched on/off, and you can drastically reconfigure the polariton Landau level spectrum by simply changing the cavity width.

Earlier this year, Charlie-Ray Mann was awarded a £5,000 prize from the Rank Prize Funds 2020.

Exeter’s Centre for Metamaterial Research and Innovation to provide novel research for £2m government offshore windfarm development

Exeter’s Centre for Metamaterial Research and Innovation (CMRI) will develop novel methods for creating conductive coating for turbine blades that adsorb radar in partnership with TWI Ltd as part of the £2M offshore windfarm development boost spearheaded by the Ministry of Defence (MOD)’s Defence and Security Accelerator (DASA) on behalf of the Department for Business, Energy and Industrial Strategy (BEIS), the Royal Air Force (RAF), and the Defence Science and Technology Laboratory (Dstl). 

In a move that could significantly boost the UK’s renewable energy growth, contracts have been awarded not only to the TWI Ltd / Exeter CMRI partnership, but also to Thales, QinetiQ, Saab, and Plextek DTS to fast-track their ideas for technologies that could mitigate the impact of windfarms on the UK’s air defence radar system.

Find out more: https://www.gov.uk/government/news/offshore-windfarm-development-boosted-by-2-million-research

 

Month 6 Presentation Prize Winners Announcement

This year, we have a joint win for our Month 6 Presentation Prize: congratulations to first year PGRs Joe Pitfield (also the winner of the Month 13 Presentation Prize)and Leanne Stanfield. They will be collecting their mystery prize soon…

The Month 6 presentations are given each May by first year PGRs, at the end of the six month mini project the students undertake before beginning their PhD project work. Feedback scores are given by their peers and by academic chairs (this year Dr Anna Katharina Ott and Dr Eric Hébrard took on these roles). Leanne’s presentation was titled ‘Graded negative index surface wave lens’ and Joe’s was titled ‘Structural stability of TMDC Heterostructures in the Presence of Water ’.  Please find their projects aims and methodology below.

Graded negative index surface wave lens

Leanne Stanfield

Aim:

Model, fabricate and characterise a graded negative index surface wave lens. One study has
already produced a free-space negative index gradient lens [1]. The aim here is to do this with surface
waves.

Methodology:

1. Develop Comsol modelling of microwave metasurfaces made of simple meta-atoms to give easy control of the surface wave index. Then vary the size of the individual elements to design a simple surface wave GRIN lens. This is a very demanding task using Comsol as the GRIN lens will need
modelling in its entirety. Once this task is complete the lens will be fabricated using wax-ink printing
and etching before being fully characterised using the XY microwave scanner.
2. Repeat the above with a more elaborate meta-atom, following on from the work of Tremain
et al [2], to give almost almost isotropic negative dispersion. Once again then vary the size of the
individual elements so that they allow for spatial control of the effective surface mode index. Then
combine such elements in a 2D metasurface structure to form a negative index GRIN lens. As for the
conventional lens this is a very demanding task using Comsol as again the GRIN lens will need
modelling in its entirety. The complete lens will then be fabricated using wax-ink printing and etching
before being fully characterised using the XY microwave scanner.

References

[1] Driscoll T, Basov DN, Starr AF, Rye PM, Nemat-Nasser S, Schurig D and Smith DR (2006) Applied
Physics Letters 88, 081101 ‘Free-space microwave focusing by negative-index gradient lens.’
[2] Tremain B’ Hooper I R, Sambles JR and Hibbins AP (2018) Scientific Reports 8, 7098 ‘Isotropic
Backward Waves Supported by a Spiral Array Metasurface.’

Structural stability of TMDC Heterostructures in the Presence of
Water

Joe Pitfield

Aim:

To explore the random structures and reorganisation of TMDC metamaterials in the presence of
water, with addition consideration of substrate effects.

Methodology:

First principles calculations combined with analytic Gibb’s Free energy arguments. In
addition, random structure prediction will be developed as a tool. This tool will require a sound
understanding of both physical and chemical environments and allow the prediction of new phases of
structures. It will need to be benchmarked against known phases.

References:

[1] F. Davies et al. TMDC Heterostructure band structure theory (submitted – preprint available)
[2] G. Schusteritsch et al. First-principles structure determination of interface materials: The NixInAs
nickelides, Phys. Rev. B, 2015.
[3] Heifets et al, BaZrO3, Phys. Rev. B, 75, 155431 (2007)

Ioannis Leontis gives oral presentation at Graphene 2020

Last month, fourth year PGR Ioannis Leontis gave an oral presentation virtually at Graphene 2020.  The event ran online from 19th-23rd October, including presentations, poster sessions and an industrial forum.

Ioannis says of the experience:

Last month, I contributed an oral presentation at the virtual conference Graphene 2020. The conference was really interesting as its programme presents very big names in the field including Nobel Laureates, such as Andre Geim, one of the fathers of graphene, and F. Duncan M. Haldane, one of the founders of the theory about topological phase transitions and the topological phases of matter. My presentation was on “Room temperature ballistic graphene p-n junctions defined by Zn metal doping”. In my talk I presented a new method for the fabrication of ultra-sharp ballistic graphene p-n junction using metal doping of graphene and a new simple characterization method of the metal induced graphene p-n junctions. My research work in this field may give a new push in the fabrication of room temperature graphene electron optic devices. In overall, my participation in such a high-level conference help me a lot in the deeper scientific understanding of my field and gave me the opportunity to make new connection as well.

Month 13 Presentation Prize Winners Announcement

This year, we have a joint win for our Month 13 Presentation Prize: congratulations to second year PGRs Joe Pitfield and Will Borrows. The prize was a £50 voucher.

The Month 13 presentations are given each October, where the PGRs give a presentation on their work to date and feedback scores are given by their peers. Joe’s presentation was titled ‘The search for new materials’ and Will’s was titled ‘Simulating heat flow in thermoacoustic devices’. Please find their abstracts below.

The search for new materials

Joe Pitfield

Abstract:

It is commonly understood that there is no universal a priori approach to predicting the nature of boundary regions between materials, nor one to enable theoretical design of novel materials within such regions. We present a developing method (RAFFLE; pseudo Random Approach For Finding (Local) Energy minima) learning from existing structural prediction methods [2][1], for the pseudo-random generation of atomic structures. Materials are characterised by decomposition of both bond angle and length, with these characterisations applied retroactively to generate new structures with profiles indicative of the isolated characteristic (in these cases, Energy of formation). This method is able to predict the existence of a series of structures of known one, two and three element systems, along with other geometries known [3] to exist for chemically similar structures (for the Transition metal dichalcogenides, both H and T phase structures predicted) and multiple stable phases identical stoichiometry structures (hexagonal and tetragonal and for Carbon) along with scopes over varying stoichiometries (HCP and FCC aluminium).


References
[1] Chris J. Pickard and R. J. Needs. ‘High-Pressure Phases of Silane’. Phys. Rev. Lett.97 (4 July 2006), p. 045504.
[2] Yanchao Wang et al. ‘Crystal structure prediction via particle-swarm optimization’. Phys. Rev. B82 (9 Sept. 2010), p. 094116.
[3] Anubhav Jain et al. ‘The Materials Project: A materials genome approach to accelerating materials innovation’. APL Materials1.1 (2013), p. 011002.

Simulating heat flow in thermoacoustic devices

Will Borrows

Abstract:

The thermoacoustic effect is a process by which sound is produced by the Joule heating of a thin film (a ‘thermophone’) with an alternating current [1]. While this effect has been known for more than a century [2], recent advances in the fabrication of nano-scale films have rejuvenated the field of thermoacoustics [3,4]. Despite this, thermophones have struggled to present themselves as an alternative to more conventional piezoacoustic speakers due to their low output efficiency.
Here we present an examination into the propagation of heat within and around a thermophone through both finite-element and finite-difference simulation of a device. We do this in order to gain a greater understanding behind the thermal processes which hinder thermophone efficiency. Of particular note is the effect of the Maxwell-Cattaneo correction to Fourier’s heat law [5], which accounts for the non-instantaneous nature of heat flow. By measuring the effect of this correction on the thermal fluctuations at the boundary between the thermophone and its surrounding medium, we are able to determine a trend for this correction. Going forward, we intend to more accurately calculate the expected sound output of a thermoacoustic device, as well as quantify the effects of this correction on the thermoacoustic efficiency.
References
[1] Ding, H. Nanoscale, 2019, v.11, p.5839-5860
[2] Preece, W. H. Proceedings of the Royal Society of London, 1880
[3] Shinoda, H. et al. Nature, 1999, v.400, p.853-855
[4] Xiao, L. et al. Nano Letters, 2008, v.8, no.12, p.4539-4545
[5] Cattaneo, C. Comptes Rendus Hebdomadaires des Seances de l’Academie des Sciences, 1958, v.145, p.431-433